Tunable multi-wavelength optical reflectors are important for a number of applications in optical telecommunications and signal processing including multiple channel optical telecommunications networks using wavelength division multiplexing (WDM). Such networks can provide advanced features, such as wavelength routing, wavelength switching and wavelength conversion, adding and dropping of channels and wavelength manipulation. Filters, such as comb filters and passband filters, are required to exclude spurious signals and to stabilize the different wavelengths. Generally such networks include optical amplifiers, such as Er-doped fiber amplifiers, with an overall bandwidth of approximately 35 nm across the ITU C-band and with a center-to-center spacing between adjacent wavelengths between 100 GHz for WDM and 25 GHz for dense WDM (DWDM).
Simple passband filters that reflect one particular wavelength are employed as cavity mirrors located at the front and rear end of the gain region in distributed Bragg reflector (DBR) lasers. These gratings tend to have a single grating period. A small degree of wavelength-tunability can be achieved by incorporating chirped gratings. However, the tuning range is in the order of 10 nm which is insufficient for optical WDM communication systems.
U.S. Pat. No. 5,325,392 discloses a grating with multiple sequentially arranged chirped regions and a wide tuning range in excess of 100 nm. The spacing between the reflectivity maxima is fixed and corresponds to the length of the repeating units. The laser can be tuned over the entire bandwidth of 45 nm by applying independently controlled currents (charge injection) to the distributed reflector region(s) and a built-in phase-adjustment region.
Conventional DBR laser grating designs typically employ deterministic gratings having a well-defined grating period which is smaller than the length of the grating, and other characteristic feature sizes, which limit their versatility. Conversely, aperiodic grating structures are defined as having repeating units with a length that exceeds the length of the grating. Aperiodic gratings have the advantage over periodic gratings that their spectral response can theoretically be selected to have any shape and form useful for the application. Several methods have been proposed to produce aperiodic grating structures, in particular for optical applications.
U.S. Pat. No. 6,141,370 discloses a grating including an array of gratings that are superimposed in an analog manner, and then subjected to a binary digitization. The process arranges grating segments of equal length and varies coefficients aj of a weighted sum of sinusoidal functions until a fit with the desired reflection peaks is obtained. The reference does not disclose how the process can be optimized.
U.S. Pat. No. 5,942,956 discloses a method for designing mode control and converter devices with an aperiodic grating. The aperiodic grating can have features of arbitrary size and arbitrary refractive index contrast. The disclosed method maximizes the power scattered into a specified mode at the output and the method is sufficiently efficient for the design of small numbers (typically tens) of high-contrast grating features. The method, however, becomes inefficient for gratings having a greater number of elements and lower contrast, as is the case for Group III-V semiconductor lasers where the refractive index contrast is typically Δn˜8×10−3 or less. A high-performance comb filter, for example, in InP may require 1,000 or more grating features which could make the scattering matrix approach difficult to manage.
U.S. Pat. No. 5,666,224 discloses a method for fabricating a non-periodic optical grating with a limited number of predefined grating lines and a limited number of predefined grating line features to keep the calculation of the matrix products of the scattering matrix manageable. The method is based on the calculation of a scattering matrix for each predefined grating line pattern or subsequence, with the scattering matrices for consecutive pairs then calculated by forming the product of the scattering matrices of the subsequences. The required pre-selection of the grating line patterns imposes limitations on the attainable spectral response of the grating.
It would therefore be desirable to develop a method for designing and producing an optical filter or grating structure that obviates the limitations of the prior art by providing grating structures with arbitrary feature sizes governed by their manufacturability, and with a predetermined spectral response, such as, for example, a comb filter with reflection maxima located at predetermined wavelengths and having predetermined relative amplitudes.